(a). Field of the Invention
The present invention relates to a pressure measuring apparatus and a pressure sensor thereof, more particularly, to a pressure measuring apparatus comprises a first resistor for sensing an external stress and a second resistor acting as a reference.
(b). Description of the Prior Arts
The working principle of pressure sensors currently available on the market, such as tire gauge, etc., is based on that the resistor buried in the pressure sensor will present different characteristics in accordance to the stress applied on the sensor. Please refer to FIG. 1A and FIG. 1B, which are respectively a side view and a sectional view schematically depicting a pressure sensor of the prior art. As seen in FIG. 1A, the pressure sensor 100 has four resistors 121-127 located on the top 115 of the substrate 110, moreover, as seen in FIG. 1B, a recess 129 is located at the center of the bottom 120 of the substrate 110, wherein the two resistors 123, 127 located on the top 115 of the substrate 110 are arranged intentionally at the places which are opposite to the two side of the recess 129.
Therefore, when an external stress is applied on the top 115 of the pressure sensor's 100 substrate 110, the external stress will be concentrated toward the area on the top 115 of the substrate 110 which is direct opposite to the recess 129 of the substrate 110, i.e. the area between the resistor 123 and the resistor 125 on the top 115 of the substrate 110. In this regard, the characteristics of the resistors 123, 125 are prone to be changed while receiving an external stress. But, on the other hand, the resistors 121, 127 are not. Thus, when an electric bridge is formed using the four resistors 121-127, the changes happened in the resistor 123 and the resistor 125 will cause a voltage difference to happen between the two nodes of the electric bridge so that the magnitude of the external stress can be measured accordingly.
Please refer to FIG. 2, which is a circuitry depicting a pressure measuring apparatus in accordance to the prior art. As seen in FIG. 2, the pressure measuring apparatus 200 is controlled by an user using the human-machine interface 210 by which the user can control the system oscillator 220 to generate a working frequency for the whole pressure measuring circuitry 200, i.e. to generate a working frequency for controlling the components in the pressure measuring circuitry 200, which are an amplifier 230, an analogue to digital converter (A/D converter) 240, a numerical converter 250 and a display controller 270. Yet, the pressure sensor 260 within the pressure measuring apparatus 200 uses the resistors 121-127 of FIG. 1 to form an electric bridge, wherein the resistors 121-127 have identical characteristics and resistor values before measuring an external stress.
Hence, when the pressure sensor 260 is connected with a reference voltage VDD for measuring an external stress, the external stress will cause the characteristics and the resistor values of the two resistor 123, 125 to change and subsequently have affect on the voltages over the two resistors 123, 125, moreover, the voltages on node 1 will also be different from the voltages on node 2. In order to use the voltage difference between the two nodes, i.e. node 1 and node 2, the voltage signal 261 of node 1 and the voltage signal 262 of node 2 are amplified by the amplifier 230 to become the signals 265, 267 that can be received by the A/D converter 240. Afterward, the foregoing analogue signals 265, 267 are converted by the A/D converter 240 to become the digital signals 269, 271 for proceeding with table lookup in the numerical converter 250.
After the digitized voltage signal 269 of node 1 and voltage signal 271 of node 2 are compared using the lookup table of the numerical converter 250, an signal 273 representing the magnitude of the external stress can be acquired using the voltage difference between the node 1 and the node 2. Finally, the external stress value signal 273 is outputted to the monitor 280 through the display controller 260 for showing the measured magnitude of the external stress on the monitor 280.
However, the pressure measuring apparatus 200 of the prior art has the following shortcomings:                1. The voltages on the node 1 and the node 2 are small voltages that will change in response to external stresses, moreover, small voltage variation will require to be amplified using the amplifier 230 before it is capable of being converted into digital signals. Thus, before converting the analogue signals of the voltages on the node 1 and the node 2, the voltages on the node 1 and the node 2 have to be amplified using the amplifier 230 to an extent that the A/D converter 240 can receive.        2. The amplifier 230 is a component having characteristics of high electricity consumption and high cost. Thus, the produces using the aforementioned pressure measuring apparatus 200 are not portable because of the characteristic of high electricity consumption will result in the apparatus 200 is incapable of using ordinary batteries, moreover, the high cost of the amplifier 230 will cause the produces using the aforementioned pressure measuring apparatus 200 to have no competitiveness in the industry.        3. Similar condition is also happening to the A/D converter 240 that the production cost thereof is high because of the high production precision requirement. Thus, the A/D converter 240 will cause the produces using the aforementioned pressure measuring apparatus 200 to suffer the same industrial competition problem.        
In view of this, the present invention provides a pressure measuring apparatus and a pressure sensor thereof using no amplifier and A/D converter so as to reduce the whole production cost of the pressure measuring circuitry.